Variable focus lens

ABSTRACT

A variable focus lens comprising a first fluid (A) and a second, non-miscible, fluid (B) in contact over a meniscus. A first electrode ( 2 ) separated from the fluid bodies by a fluid contact layer ( 10 ), and a second electrode ( 12 ) in contact with the first fluid to cause an electrowetting effect whereby the shape of the meniscus is altered. The fluid contact layer has a substantially cylindrical inner wall.

This invention relates to a variable focus lens comprising a first fluidand a second fluid which are in contact over a meniscus and to a methodof operating such a variable focus lens. The shape of the meniscus canbe controlled by a voltage.

A fluid is a substance that alters its shape in response to any force,that tends to flow or to conform to the outline of its chamber, and thatincludes gases, liquids and mixtures of solids and liquids capable offlow.

The meniscus between the first fluid and the second fluid is calledconcave, if the meniscus is hollow as seen from the second fluid. If thefirst fluid is regarded as a lens, this lens would normally calledconcave if the meniscus is concave according to the definition in theprevious sentence.

A variable focus lens having such an arrangement is described inInternational patent application WO 99/18456. In this arrangement, thelens comprises a chamber filled with a conductive first liquid, adroplet of an insulating, non-miscible second liquid being held in asurface zone of the chamber wall by a fluid contact layer applied on thewall. The fluid contact layer positions the droplet because part of thefluid contact layer is hydrophobic and an adjacent part is hydrophilic.Application of a voltage to electrodes in the chamber causes therefracting upper surface or meniscus of the droplet to become moreconvex. In one embodiment, the hydrophobic and hydrophilic parts of thefluid contact layer are arranged along a cylindrical surface, the sidesof the droplet being positioned axially along the cylindrical surface,and thereby centred, by the hydrophilic part when no voltage is appliedand by a series of electrodes along the sides of the cylinder when avoltage is applied. Such a lens is complex to manufacture and,particularly in the cylindrical configuration, requires a relativelyhigh voltage in order to alter the lens characteristics of the droplet,which can cause premature degradation of the lens when used over aperiod of time.

A further variable focus lens having such an arrangement is described inthe international patent application WO 00/58763. The proposed means forcentring a droplet of insulating liquid is a bell-mouthed recess formedof an insulating layer in an adjustable lens. The sides of the recessare arranged so as to keep the droplet centred within the recess and toprovide a convex refracting surface on the droplet. The recess is shapedsuch that the manufacture of such a lens remains relatively complex, andsince the base of the recess is formed of the same material as the sidesof the recess, such material must be chosen to be transparent if thelens is to be operative.

In accordance with the present invention, there is provided a variablefocus lens including

-   -   a substantially cylindrical fluid chamber having a cylinder wall        and an axis, the fluid chamber including a first fluid (A) and        an axially displaced second fluid (B), the fluids being        non-miscible, in contact over a meniscus and having different        indices of refraction,    -   a fluid contact layer arranged on the inside of the cylinder        wall,    -   a first electrode separated from the first fluid and second        fluid by the fluid contact layer,    -   a second electrode acting on the second fluid,    -   the fluid contact layer having a wettability by the second fluid        which varies under the application of a voltage between the        first electrode and the second electrode, such that the shape of        the meniscus varies in dependence on the said voltage,    -   wherein the wettability of the fluid contact layer by the second        fluid is substantially equal on both sides of the intersection        of the meniscus with the contact layer when no voltage is        applied between the first and second electrodes.

The equal wettability of the fluid contact layer on both sides of theintersection allows a larger movement of the meniscus and, as aconsequence, a greater change in curvature of the meniscus. It allows aconcave meniscus to become convex or vice versa.

In a preferred embodiment the lens is arranged to produce a meniscusshape which is concave, the shape becoming less concave at increasingmagnitude of voltage applied between the first and second electrodes.With the fluid contact layer substantially cylindrical, the tendency ofthe first fluid to wet the fluid contact surface can be used to producethe concave meniscus shape, and furthermore, relatively low voltages canbe used to vary the meniscus shape to alter the power of the lens.Thereby, a desired range in lens power may be produced without theapplication of excess voltage.

By using a substantially cylindrical inner surface of the fluid contactlayer and arranging the lens to produce a concave meniscus shape, therange in lens power of the lens can be improved without the applicationof excess voltage. At sufficiently high magnitude of voltage the shapeof the meniscus may become convex. Application of excess voltage canlead to the charging of the fluid contact layer, which has been found tocause degradation of the layer, leading to a significant reduction inthe useful lifetime of the lens.

A substantially cylindrical inner surface for the fluid contact layermay be produced without the need for complex processing techniques. Inparticular, such an inner surface shape may be produced by dip coatingof a cylindrical electrode, which is a relatively reliable andinexpensive procedure. The fluid contact layer is furthermore preferablyof a uniform thickness so as to provide a reliable refractive behaviourof the meniscus throughout the adjustable range of the lens. Again, sucha uniform fluid contact layer can be readily produced by dip coating acylindrical electrode element.

A second aspect of the invention relates to a method of operating avariable focus lens including a substantially cylindrical fluid chamberhaving a cylinder wall, the fluid chamber including a first fluid (A)and an axially displaced second fluid (B), the fluids beingnon-miscible, in contact over a meniscus and having different indices ofrefraction, a fluid contact layer arranged on the inside of the cylinderwall, a first electrode separated from the first fluid and second fluidby the fluid contact layer, a second electrode acting on the secondfluid, the wettability of the fluid contact layer by the second fluidbeing substantially equal on both sides of the intersection of themeniscus with the contact layer when no voltage is applied between thefirst and second electrodes, the wettability of the fluid contact layerby the second fluid varying under the application of a voltage betweenthe first electrode and the second electrode, the method comprisingcontrolling the said voltage to change the shape of the meniscus.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, wherein:

FIGS. 1 to 3 show an adjustable lens in accordance with an embodiment ofthe invention in schematic cross section;

FIG. 4 shows an image capture device in accordance with an embodiment ofthe invention in schematic cross section; and

FIG. 5 shows an optical scanning device in accordance with an embodimentof the invention in schematic cross section.

FIGS. 1 to 3 show a variable focus lens comprising a cylindrical firstelectrode 2 forming a capillary tube, sealed by means of a transparentfront element 4 and a transparent back element 6 to form a fluid chamber5 containing two fluids. The electrode 2 may be a conducting coatingapplied on the inner wall of a tube.

In this embodiment the two fluids consist of two non-miscible liquids inthe form of an electrically insulating first liquid A, such as asilicone oil or an alkane, referred to herein further as “the oil”, andan electrically conducting second liquid B, such as water containing asalt solution. The two liquids are preferably arranged to have an equaldensity, so that the lens functions independently of orientation, i.e.without dependence on gravitational effects between the two liquids.This may be achieved by appropriate selection of the first liquidconstituent; for example alkanes or silicon oils may be modified byaddition of molecular constituents to increase their density to matchthat of the salt solution.

Depending on the choice of the oil used, the refractive index of the oilmay vary between 1.25 and 1.60. Likewise, depending on the amount ofsalt added, the salt solution may vary in refractive index between 1.33and 1.48. The fluids in this embodiment are selected such that the firstfluid A has a higher refractive index than the second fluid B.

The first electrode 2 is a cylinder of inner radius typically between 1mm and 20 mm. The electrode 2 is formed from a metallic material and iscoated by an insulating layer 8, formed for example of parylene. Theinsulating layer has a thickness of between 50 nm and 100 μm, withtypical values between 1 μm and 10 μm. The insulating layer is coatedwith a fluid contact layer 10, which reduces the hysteresis in thecontact angle of the meniscus with the cylindrical wall of the fluidchamber. The fluid contact layer is preferably formed from an amorphousfluorocarbon such as Teflon™ AF1600 produced by DuPont™. The fluidcontact layer 10 has a thickness of between 5 nm and 50 μm. The AF1600coating may be produced by successive dip coating of the electrode 2,which forms a homogeneous layer of material of substantially uniformthickness since the cylindrical sides of the electrode are substantiallyparallel to the cylindrical electrode; dip coating is performed bydipping the electrode whilst moving the electrode in and out of thedipping solution along its axial direction. The paralyne coating may beapplied using chemical vapour deposition. The wettability of the fluidcontact layer by the second fluid is substantially equal on both sidesof the intersection of the meniscus 14 with the fluid contact layer 10when no voltage is applied between the first and second electrodes.

A second, annular electrode 12 is arranged at one end of the fluidchamber, in this case, adjacent the back element. The second electrode12 is arranged with at least one part in the fluid chamber such that theelectrode acts on the second fluid B.

The two fluids A and B are non-miscible so as to tend to separate intotwo fluid bodies separated by a meniscus 14. When no voltage is appliedbetween the first and second electrodes, the fluid contact layer has ahigher wettability with respect to the first fluid A than the secondfluid B. Due to electrowetting, the wettability by the second fluid Bvaries under the application of a voltage between the first electrodeand the second electrode, which tends to change the contact angle of themeniscus at the three phase line (the line of contact between the fluidcontact layer 10 and the two liquids A and B). The shape of the meniscusis thus variable in dependence on the applied voltage.

Referring now to FIG. 1, when a low voltage V₁, e.g. between 0 V and 20V, is applied between the electrodes the meniscus adopts a first concavemeniscus shape. In this configuration, the initial contact angle θ₁between the meniscus and the fluid contact layer 10, measured in thefluid B, is for example approximately 140°. Due to the higher refractiveindex of the first fluid A than the second fluid B, the lens formed bythe meniscus, here called meniscus lens, has a relatively high negativepower in this configuration.

To reduce the concavity of the meniscus shape, a higher magnitude ofvoltage is applied between the first and second electrodes. Referringnow to FIG. 2, when an intermediate voltage V₂, e.g. between 20 V and150 V, depending on the thickness of the insulating layer, is appliedbetween the electrodes the meniscus adopts a second concave meniscusshape having a radius of curvature increased in comparison with themeniscus in FIG. 1. In this configuration, the intermediate contactangle θ₂ between the first fluid A and the fluid contact layer 10 is forexample approximately 100°. Due to the higher refractive index of thefirst fluid A than the second fluid B, the meniscus lens in thisconfiguration has a relatively low negative power

To produce a convex meniscus shape, a yet higher magnitude of voltage isapplied between the first and second electrodes. Referring now to FIG.3, when a relatively high voltage V₃, e.g. 150 V to 200 V, is appliedbetween the electrodes the meniscus adopts a meniscus shape in which themeniscus is convex. In this configuration, the maximum contact angle θ₃between the first fluid A and the fluid contact layer 10 is for exampleapproximately 60°. Due to the higher refractive index of the first fluidA than the second fluid B, the meniscus lens in this configuration has apositive power.

Note that, whilst achieving the configuration of FIG. 3 is possibleusing a relatively high power, it is preferred in a practical embodimentthat a device including the lens as described is adapted to use only lowand intermediate powers in the ranges described, that is to say that thevoltage applied is restricted such that the electrical field strength inthe insulating layer is smaller than 20 V/μm, and excessive voltagescausing charging of the fluid contact layer, and hence degradation ofthe fluid contact layer, are not used.

Note furthermore that the initial, low voltage, configuration will varyin dependence on the selection of the liquids A and B, in dependence ontheir surface tensions). By selecting an oil with a higher surfacetension, and/or by adding a component, such as ethylene glycol, to thesalt solution which reduces its surface tension, the initial contactangle can be decreased; in this case the lens may adopt a low opticalpower configuration corresponding to that shown in FIG. 2, and anintermediate power configuration corresponding to that shown in FIG. 3.In any case, the low power configuration remains such that the meniscusis concave, and a relatively wide range of lens powers can be producedwithout using an excessive voltage.

Although the fluid A has a higher refractive index than fluid B in theabove example, the fluid A may also have a lower refractive index thanfluid B. For example, the fluid A may be a (per)fluorinated oil, whichhas a lower refractive index than water. In this case the amorphousfluoropolymer layer is preferably not used, because it might dissolve influorinated oils. An alternative fluid contact layer is e.g. a paraffincoating.

FIG. 4 illustrates a variable focus image capture device including alens in accordance with an embodiment of the present invention. Elementssimilar to that described in relation to FIGS. 1 to 3 are provided withthe same reference numerals, incremented by 100, and the previousdescription of these similar elements should be taken to apply here.

The device includes a compound variable focus lens including acylindrical first electrode 102, a rigid front lens 104 and a rigid rearlens 106. The space enclosed by the two lenses and the first electrodeforms a cylindrical fluid chamber 105. The fluid chamber holds the firstand second fluids A and B. The two fluids touch along a meniscus 114.The meniscus forms a meniscus lens of variable power, as previouslydescribed, depending on a voltage applied between the first electrode102 and the second electrode 112. In an alternative embodiment, the twofluids A and B have changed position.

The front lens 104 is a convex-convex lens of highly refracting plastic,such as polycarbonate or cyclic olefin copolymer, and having a positivepower. At least one of the surfaces of the front lens is aspherical, toprovide desired initial focusing characteristics. The rear lens element106 is formed of a low dispersive plastic, such as COC (cyclic olefincopolymer) and includes an aspherical lens surface which acts as a fieldflattener. The other surface of the rear lens element may be flat,spherical or aspherical. The second electrode 112 is an annularelectrode located to the periphery of the refracting surface of the rearlens element 106.

A glare stop 116 and an aperture stop 118 are added to the front of thelens. A pixellated image sensor 120, such as a CMOS sensor array, islocated in a sensor plane behind the lens.

An electronic control circuit 122 drives the meniscus lens, inaccordance with a focus control signal, derived by focus controlprocessing of the image signals, so as to provide an object range ofbetween infinity and 10 cm. The control circuit controls the appliedvoltage between a low voltage level, at which focusing on infinity isachieved, and higher voltage levels, when closer objects are to befocused. When focusing on infinity, a concave meniscus with a contactangle of approximately 140° is produced, whilst when focusing on 10 cm,a concave meniscus with a contact angle of approximately 100° isproduced.

The conducting second fluid, the insulating layer and the secondelectrode form an electrical capacitor, the capacitance of which dependson the position of the meniscus. The capacitance can be measured using aconventional capacitance meter. The optical strength of the meniscuslens can be determined from the measured value of the capacitance.

The lens is configured such that a low, non-zero, voltage is applied tofocus the lens on an object at infinity (parallel incoming rays), so asto provide the capability to focus on infinity within reasonablemanufacturing tolerances; if on the other hand the lens were to beconfigured such that focusing on infinity occurred when zero voltage isapplied, more strict manufacturing tolerances would have to be applied.

The front lens element 104 is preferably formed as a single body with atube holding the electrode 102 on its inner surface and closed off bythe rear lens 106 to form a sealed unit. The second lens element 106 maybe extended, in relation to that shown in FIG. 4, and the flat rearsurface of the lens element 106 may be replaced by an angled mirrorsurface, preferably angled at 45°, to allow the image sensor 120 to beplaced below the lens, in order to reduce the dimensions of the lens.

The fluid chamber 105 may be provided with an expansion chamber toaccommodate volume changes due to thermal expansion of the fluids. Theexpansion chamber may be a flexible membrane in one of the walls of thefluid chamber.

The inner surfaces of the front lens 104 and the rear lens 106 may becoated with a protective layer to avoid incompatibility of the materialfrom which the lenses are made with the fluids A and B. The protectivelayer may also have anti-reflection characteristics.

FIG. 5 shows elements from an optical scanning device containing a lensin accordance with an embodiment of the invention. The device is forrecording and/or playback from an optical disk 206, for example a duallayer digital video recording (DVR) disk (see for instance the articleby K. Schep, B. Stek, R. van Woudenberg, M. Blum, S. Kobayashi, T.Narahara, T. Yamagami, H. Ogawa, “Format description and evaluation ofthe 22.5 GB DVR disc”, Technical Digest, ISOM 2000, Chitose, Japan, Sep.5-8, 2000). The device includes a compound objective lens, for instancehaving a numerical aperture of 0.85, including a rigid front lens 202and a rigid rear lens 204, for instance as described in Internationalpatent application WO 01/73775, for focusing the incoming collimatedbeam, for instance having a wavelength of 405 nm, consisting ofsubstantially parallel rays, to a spot 208 in the plane of aninformation layer currently being scanned.

In dual layer DVR disks the two information layers are at depths of 0.1mm and 0.08 mm; they are thus separated by typically 0.02 mm. Whenrefocusing from one layer to the other, due to the difference ininformation layer depth, some 200 mλ of unwanted spherical wavefrontaberration arises, which needs to be compensated. One way to achievethis is to change the vergence of the incoming beam using a mechanicalactuator, for example moving a collimator lens in the device, which isrelatively expensive. Another approach is to use a switchable liquidcrystal cell, which is also a relatively expensive solution.

In this embodiment, a switchable variable focus lens 200 similar to thatdescribed in relation to FIGS. 1 to 3 is used. In this embodiment, theoil chosen is polydimethyl (8-12%)-phenylmethylsiloxane copolymer, and asalt water solution is used as the conducting liquid. Each of theliquids, when the lens 200 is arranged with a planar meniscus, has athickness of approximately 1 mm.

The device includes an electronic control circuit 222 for applying oneof two selected voltages to the electrodes of the lens 200 in dependenceon the information layer currently being scanned. In one configuration,during the scanning of the information layer depth of 0.08 mm, arelatively low selected voltage is applied to produce a meniscuscurvature of radius R=21.26 mm. In the other configuration, during thescanning of the information layer depth of 0.1 mm, a relatively highselected voltage is applied to produce a planar meniscus curvature. As aresult, the root mean square value of the wavefront aberration can bereduced from 200 mλ to 18 mλ. Note that a similar effect can be obtainedusing different combinations of meniscus curvatures, since only avariation in lens power is required; furthermore the difference in lenspower can also be achieved with larger movements in the meniscus bymaking the refractive indices of the two liquids more similar.

Note, in relation to all the above embodiments, the electrode is itselfpreferably cylindrical, but some variation from a perfect cylinder ispossible, e.g. slightly conical. However, the cylinder should preferablyremain substantially cylindrical, namely where the fluid contact layerhas a linear cross section, i.e. the layer forms straight lines in across section of the cylinder, where the axis of the cylinder lies inthe cross section. The linear cross section should be parallel to theaxis of the electrode at least to within 10 degrees, more preferably atleast to within 1 degree. A cylindrical electrode can be made usingconventional, cheap tubing having a cross section which is parallel tothe axis within 0.1 degree and a smooth inner wall on which the variouslayers can be deposited. The possibility to use such tubing gives thelens according to the invention a cost advantage. The fluid contactlayer may itself not be perfectly linear; however any non-linearity ispreferably limited such that the non linearity causes a difference inradial extent less than one tenth, more preferably less than onetwentieth, of the axial extent of the electrode.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. Forexample, the first fluid may consist of a vapour rather than aninsulating liquid. The second fluid may be a fluid having a lowersurface tension than the first fluid. In that case the shape of themeniscus at low applied voltages will be convex.

It is to be understood that any feature described in relation to oneembodiment may also be used in other of the embodiments.

Furthermore, equivalents and modifications not described above may alsobe employed without departing from the scope of the invention, which isdefined in the accompanying claims.

1. A variable focus lens including a substantially cylindrical fluidchamber having a cylinder wall, the fluid chamber including a firstfluid (A) and an axially displaced second fluid (B), the fluids beingnon-miscible, in contact over a meniscus (14) and having differentindices of refraction, a fluid contact layer (10) arranged on the insideof the cylinder wall, a first electrode (2) separated from the firstfluid and second fluid by the fluid contact layer, a second electrode(12) acting on the second fluid, the fluid contact layer having awettability by the second fluid which varies under the application of avoltage between the first electrode and the second electrode, such thatthe shape of the meniscus varies in dependence on the said voltage,wherein the wettability of the fluid contact layer by the second fluidis substantially equal on both sides of the intersection of the meniscuswith the contact layer when no voltage is applied between the first andsecond electrodes.
 2. A lens according to claim 1, wherein the innersurface of the fluid contact layer has a linear cross-section, andwherein the linear cross section is parallel to the axis of thesubstantially cylindrical shape of the surface to within 10 degrees. 3.A lens according to claim 1, wherein the first fluid includes aninsulating liquid and the second fluid includes a conducting liquid. 4.A lens according to claim 1, wherein the first fluid includes a vapourand the second fluid includes a conducting liquid.
 5. A lens accordingto claim 1, wherein the lens is arranged to produce a meniscus shapewhich is concave when viewed from the second fluid, the shape becomingless concave at increasing magnitude of voltage applied between thefirst and second electrodes.
 6. A lens according to claim 1, wherein thefluid contact layer is a substantially homogeneous layer of uniformthickness.
 7. A lens according to claim 1, wherein said first electrodeis substantially cylindrical.
 8. A lens according to claim 1, whereinsaid first fluid has a larger refractive index than said second fluidand wherein the lens is a compound lens comprising at least one fixedlens element (104) providing a positive lens power, such that thecompound lens has a positive lens power when the meniscus is convex inrelation to the first fluid.
 9. An optical device comprising a lensaccording to claim 1, the device comprising means defining a focusingplane (120) wherein the lens is arranged such that when radiationconsisting of parallel rays is input and a non-zero voltage is appliedbetween the first and second electrodes, the radiation is focused on thefocusing plane.
 10. An image capture device comprising a lens accordingto claim
 1. 11. An optical scanning device for scanning an opticalrecord carrier, comprising a lens according to claim
 1. 12. An opticalscanning device according to claim 12, wherein said lens is arranged tocorrect for spherical aberrations arising during the scanning ofdifferent information layer depths in optical record carriers beingscanned.
 13. A method of operating a variable focus lens including asubstantially cylindrical fluid chamber having a cylinder wall, thefluid chamber including a first fluid (A) and an axially displacedsecond fluid (B), the fluids being non-miscible, in contact over ameniscus (14) and having different indices of refraction, a fluidcontact layer (10) arranged on the inside of the cylinder wall, a firstelectrode (2) separated from the first fluid and second fluid by thefluid contact layer, a second electrode (12) acting on the second fluid,the wettability of the fluid contact layer by the second fluid beingsubstantially equal on both sides of the intersection of the meniscuswith the contact layer when no voltage is applied between the first andsecond electrodes, the wettability of the fluid contact layer by thesecond fluid varying under the application of a voltage between thefirst electrode and the second electrode, the method comprisingcontrolling the said voltage to change the shape of the meniscus.
 14. Amethod according to claim 13 wherein said method comprises varying saidvoltage to produce a meniscus shape which is concave when viewed fromthe second fluid.
 15. A method according to claim 13, wherein saidmethod further comprises varying said voltage to produce a meniscusshape which is convex when viewed from second fluid.
 16. A methodaccording to claim 15, wherein said meniscus has a contact angle withthe fluid contact layer of between 100 and 140 degrees.